Make sure to scrape the sides and bottom of bowl so everything is mixed.

Beat on high speed 3 minutes, scraping sides of bowl occasionally.

Spread about three cups of batter evenly in prepared pan.

Mix the cocoa and baking soda into remaining batter until well blended.

Drop heaping tablespoons of the chocolate batter randomly on the peanut butter batter.

Marble the batters by pulling a knife through the batters in S shaped curves in one direction, then turn and do S shaped curves across the lines done previously.

Bake for 40 to 45 minutes or until done.

Cool completely in pan on wire rack.

About 1 hour.

Stir together icing sugar and 1/3 cup butter in medium bowl.

Stir in vanilla, milk and 1/4 cup peanut butter.

Beat until smooth and spreadable.

Spread the icing mixture evenly on the cake.

Either drizzle the chocolate topping evenly over the top of the icing or put it on in stripes or teaspoonfulls and marble it using a knife drawn through the icing, first in one drirection at 1" intervals and then in the other.